1. Field of the Invention
The present invention relates to a solid-state image pickup device.
2. Description of the Related Art
Various types of solid-state image pickup devices, such as a metal oxide semiconductor (MOS) transistor type, a charge coupled device (CCD) type, and the like, have previously been proposed and put into practical use. As the MOS type, there is an amplification type solid-state image pickup device in which pixels of an amplification type solid-state imaging device (active pixel sensor (APS)) configuration having an amplification type drive transistor are provided in a pixel signal generation means for generating a pixel signal corresponding to a signal charge generated by a charge generation means. Many (complementary) metal oxide semiconductor ((C)MOS) type solid-state image pickup devices are configured as described above.
Furthermore, column analog-to-digital (AD) converter (ADC) type solid-state image pickup devices in which an AD conversion function is embedded in a column unit have also been developed and commercialized. In addition to 1. Successive comparison AD conversion type, 2. Single slope AD conversion type, 3. Cyclic AD conversion type, and the like as AD conversion types, a column ADC type solid-state image pickup device (see Japanese Unexamined Patent Application, First Publication No. 2006-270293) has been proposed in which a time-to-digital converter (TDC) having a “voltage-frequency conversion” function according to a signal from a pixel is provided in the column unit. As disclosed in Japanese Unexamined Patent Application, First Publication No. 2006-270293, it is possible to comparatively easily AD-convert a signal from a pixel at a high signal-to-noise ratio (SNR) by use of the TDC.
Here, in general, the SNR by which image quality of the (C)MOS type solid-state image pickup device is determined will be described. A signal S is a value of a voltage into which a charge accumulated in a pixel is converted by a floating diffusion (FD) unit. 1. Photon shot noise, 2. l/f noise, 3. Thermal noise, and 4. Circuit noise due to a potential fluctuation of a power supply/ground exist as a noise N. In general, a key point for improving the SNR is to reduce the l/f noise, the thermal noise, and the circuit noise due to the potential fluctuation of the power supply/ground.
It is common to increase the size of a transistor or narrow a sampling frequency so as to reduce the l/f noise. Also, it is common to narrow a pass band of a signal so as to reduce the thermal noise. Also, it is common to reduce (the influence of) a potential fluctuation by decreasing a resistance value of wiring so as to reduce the circuit noise due to the power supply/ground fluctuation.
If the TDC is applied to the column ADC type solid-state image pickup device, a streak phenomenon (lateral noise) occurs due to a fluctuation in a power supply voltage supplied to a voltage controlled oscillator (VCO) or ring delay line (RDL) circuit, which is a frequency conversion unit constituting a core of the TDC.
A propagation delay time of an inverting circuit constituting the VCO circuit or RDL circuit largely depends on the power supply voltage (a potential difference supplied to each of an upper power supply terminal and a lower power supply terminal connected to the inverting circuit). In the streak phenomenon, the potential fluctuation of the power supply/ground appears as noise.
Here, as shown in FIG. 7, it is assumed that a pixel signal A is large in a column A, a value of a current Ia flowing through a frequency converter is large, a pixel signal B is small in an adjacent column B and a value of a current Ib flowing through a frequency converter is small. Incidentally, R is the wiring resistance, and VSS is the ground. The influence of a voltage drop (ΔV≈R×Ia) due to the current Ia (>Ib) flowing through the column A typically appears in a terminal b, and a ground potential fluctuates. In particular, for example, because there is one TDC per column in the column ADC type solid-state image pickup device, the entire fluctuation increases even when there is a small potential fluctuation of a power supply/ground per TDC, and becomes a problem as noise.